Differentiation in insect epidermis is controlled by ecdysone and juvenile hormone (JH). At metamorphosis corpora allata, the source of JH are inactivated and the endogenous JH is degraded by JH-specific enzymes. The correlation, if any, between the appearance of JH metabolizing enzymes and the "pupal potential" of larval epidermis is being investigated. JH esterase activity in larvae, pupae and adults of Galleria was measured using tritiated C18 Cecropia JH as Substrate. JH esterase activity is low in penultimate and early (1 to 3 days) last larvae. The enzyme activity in days 4 and 5 larvae is six fold higher than in day 3 larvae. It decreases till pupation and is again high in day 1 pupae. In pharate adults, the enzyme activity decreases and remains low in female moths. In males however, it is higher. Developmental capacity of larval epidermis was monitored by injecting high doses of ecdysterone (10 micrograms/larva). Day four last instar larva is the earliest stage at which epidermis expresses pupal characters. Whether the simultaneous appearance of "pupal potential" and JH esterases in day 6 last larvae is coincidental or is causally correlated is being explored, as are also the factors that induce JH esterase at specific developmental stages.